1. Field of the Invention
The present invention relates to a position-sensing detector and in particular to a method of determining the position of a fast pulsing laser incident on the detector""s surface for the purpose of auto-aligning the laser.
2. Description of the Prior Art
Many laser applications involve precise alignment of the laser beam. Auto-alignment techniques all depend on accurately determining the laser beam""s position in two dimensions (2D). One detector used for continuous wave (CW) lasers is a silicon photodiode called a position-sensing detector (PSD). It provides an analog output directly proportional to the position of a laser beam on the detector""s active area independent of intensity fluctuations.
A one dimensional (1D) position sensing detector, shown in FIG. 1, consists of an n-type silicon substrate with two resistive layers separated by a p-n junction. The front side has an ion implanted p-type resistive layer with two contacts at opposite ends. The backside has an ion implanted n-type resistive layer with two contacts at opposite ends placed orthogonal to the contacts on the front side. The electrodes are placed at opposite ends of the p-type resistive layer. A laser beam within the spectral range of silicon will generate a photocurrent that flows from the incident point through the resistive layers to the electrodes. Since the resistivity of the ion implanted layer is extremely uniform, the photogenerated current at each electrode is inversely proportional to the distance between the incident spot of light and the electrodes. The centroid of power density of the spot is thereby tracked by the PSD (FIG. 2). See On-Trak Photonics, Inc. at www.on-trak.com/theory.html.
A two-dimensional PSD has four terminals and can output the X, Y coordinates of a laser spot on its detector surface (See FIG. 3). The photoelectric current generated by the incident laser beam flows through the device and can be seen as two input currents (X1 and X2) and two output currents (Y1 and Y2). The distribution of the output currents show the beam position of one dimension (Y) and the distribution of the input currents show the beam position of the second dimension (X) according to the equations given in FIG. 3.
PSDs and their accompanying electronics have the capability to run at less than 16 kHz. This means that commercially available systems are for use with basically CW lasers. These PSDs have sufficient response times to detect fast pulsing lasers. However, the electronics for determining the beam position for fast pulsing lasers (in the range of 100 nanosecond wide pulses and 10 kHz repetition rates) are not currently available. The present invention fulfills this need.
The present invention is an electronic circuit that interfaces with a silicon position-sensing detector to determine the position of a fast pulsing laser on the order of 100 ns wide pulses and repetition rates of 10 kHz. The PSD, designed as a continuous analog position sensor for CW lasers, is thereby converted to a position sensor for fast pulsed lasers.